Prime minister's lack of leadership on green issues among concerns raised by head of charity that helped rebrand party.

The head of the charity that helped to arrange David Cameron's memorable husky photoshoot in the Arctic, launching the Conservatives' rebranding as the nice-not-nasty party, has warned that the PM's lack of leadership on environment issues risks "retoxifying" their image.

The striking images of Cameron posing on the ice with huskies on the way to visiting a melting glacier in 2006 marked a turning point for the Conservatives, who had been seen by many voters as uncaring. After the pictures appeared across newspapers and TV back home, Cameron's image-maker and policy guru, Steve Hilton, is said to have received a text from an ally back home: "Simply brilliant – that was worth a thousand speeches."

Reflecting on the 2006 trip, David Nussbaum, the chief executive of WWF UK, said: "What we were most encouraged about was it was part of his detoxification of the Conservative brand. This was a symbolic indication that the Conservative party had changed, [it] wasn't any longer the 'nasty party'."

Almost six years later, however, and with Cameron having been prime minister for nearly two years, Nussbaum's first full verdict on how the Conservative leader has lived up to that trip's promise is mixed, at best.

While on many policies there has been welcome progress, Nussbaum believes there have been too many caveats – some of them large – that have undermined those policies, and too much inconsistency in decisions and speeches from senior Tories.

"Clearly at the moment the polls are pretty positive, but we know polls can move dramatically and quickly," said Nussbaum, whose predecessor as head of WWF UK, Robert Napier, accompanied Cameron, Hilton and the now climate minister Greg Barker to the Norwegian archipelago of Svalbard. He said: "The long-term future of the Conservative party David Cameron is trying to lead is the party which continues to embrace people for whom environmental sustainability, care for the natural world, thinking about what we are leaving our future generations, those are deeply held values.

"The risk of retoxification would be very serious … to the range of people who are potential supporters of a Conservative government."